Remote sensing acquires information about an object or a place without making physical contact with the object or the place. Aerial sensor technologies are generally used to detect and classify objects on a surface (e.g. of the earth), in the atmosphere, and in water by means of propagated signals such as electromagnetic signals emitted from aircraft or satellites. Remote sensing is generally done via passive or active remote sensing. Passive sensors detect natural radiation such as sunlight that is emitted or reflected by the object or the place being observed. Active collection emits a signal that is reflected or backscattered from the object or the place, and detected by a sensor. A time delay between emission and return of the signal may be measured to determine a location, height, speed and direction of an object. Examples of remote sensors include film photography, infrared, charge-coupled devices, radiometers, RADAR and light detection and ranging (LiDAR).
Remote sensing allows collection of data in dangerous or inaccessible areas, and can replace costly or slow ground data collection, while ensuring that objects or places are not disturbed. Remote sensing applications may comprise applications such as monitoring deforestation, vegetation rates, erosion, pollution, forestry, weather, land use, measuring sea ice, depth sounding in water, collection of data about dangerous areas, natural resource management, land usage and conservation, stand-off collection on border areas, city planning, archaeological investigations, military observation, geomorphological surveying, and other remote sensing applications.